How many were deterred?
Number of bombings prevented: Zero.How many would have happened without screening? We don't know. Your argument is sort of like Obama's counting of jobs saved.
The federal government does not fulfill the requirement they are under, so of course I've no answer to your questions. It is up to the government to demonstrate the threat, that the means being used to intercept this threat is justified, and that the actions taken has shown results.
This is just like police checkpoints - the police can't just set one up for no reason, they must demonstrate first that there is a threat (DUI rates, reckless driving rates, etc) in a particular stretch of road before they can put up a checkpoint, and then, they are subject to rules - the checkpoint can't be on the only road in an area, there must be signs up to permit drivers to avoid the checkpoint (HA), and may not prosecute anything that isn't in plain sight, or have a reasonable suspicion and permission to conduct a search.
The theory is that you, as a driver, chose to enter the checkpoint. The reality is that if you turn around from a checkpoint, you're going to get pulled over, but there's at least a modicum of attention paid to the 4th Amendment.
These airport checkpoints do not limit themselves to plain sight, use a search procedure that isn't permitted unless a person's actually arrested and processed into a jail facility, and they're spreading to courthouses. If they weren't prosecuting these minor crimes that are uncovered from illegal searches, I'd be less concerned, but they are prosecuting them, and enhancing them with vague 'secure area' regulations.
Just as Obama had to justify counting jobs saved that weren't saved, the TSA has to justify violating our rights each day. Along with the many courthouses and other public buildings these scanners are going to be installed in. Because after all, once you've sold to all the TSA checkpoints, you gotta keep selling your product, right?
Maybe they'll put one in your local school to 'counter gang fights.' I'm sure there's a pedo working on making an automatic colorization software for those images right now...
So, the previous security measures were ineffective and we therefore had all kinds of terrorists attacks? Can you name them, I am not familiar with those attacks.